Mechanical-bond-protected stable bisradicals

ABSTRACT

Disclosed herein are compositions comprising air-stable radical [2]catenanes and method of making the same. The [2]catenane comprises a first macrocyclic ring mechanically interlocked with a second macrocyclic ring where each of the first macrocyclic ring and the second macrocyclic ring comprise an alternating cyclic arrangement of a first unsubstituted or substituted 4,4′-bipyridinium (BIPY) subunit, a first unsubstituted or substituted phenylene subunit, a second unsubstituted or substituted BIPY subunit, and a second unsubstituted or substituted phenylene subunit forming a macrocycle.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims benefit of priority to United StatesApplication Ser. No. 62/994,778, filed Mar. 25, 2020, the contents ofwhich is incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

In recent decades, the development of new stable organic radicals hasbecome a topic for extensive investigations^(1,2) because of theirunique optical,³ electronic,⁴ and magnetic⁵ properties. Most organicradicals are unstable under ambient conditions and dimerize⁶ quickly-toform new covalent bonds—or become oxidized/reduced under ambientconditions, making their isolation and characterization demanding tasks.In general, there are several common strategies for enhancing theair-stability of organic radicals, such as (i) increasing the sterichindrance around the radical center⁷ in order to prevent dimerization,(ii) introducing electron-withdrawing groups to lower the LUMO energylevel⁸ in order to enhance resistance to oxidation by O₂ and H₂O, and(iii) recovering aromaticity⁹, amidst other exmaples¹. However, due tothe difficulty in stabilizing organic radicals, new compositions andmethods are needed.

BRIEF SUMMARY OF THE INVENTION

Disclosed herein are compositions comprising air-stable radical[2]catenanes and methods of making the same. One aspect of the inventionincludes compositions comprising a [2]catenane. The [2]catenanecomprises a first ring mechanically interlocked with a second ring or asalt thereof. In some embodiments, the [2]catenane is an air-stablebisradical hexacationic state, an air-stable monoradical heptacationicstate, or the mixture of both states. In some embodiment, the firstring, the second ring, or both the first ring and the second ring aremCBPQT. In a particular embodiment, both the first ring and the secondring are mCBPQT. In another embodiment, only one of the first ring andthe second ring are mCBPQT.

The compositions described herein may have positive reductionpotentials. Suitably, the composition has an E_(red1) greater than +0.50V versus Ag/AgCl and/or the composition has an E_(red2) greater than+0.25 V versus Ag/AgCl.

The composition described herein may have a near infrared absorptionband longer than 1200 nm. Suitably, the near infrared absorption bandlonger than 1400 nm, 1600 nm, or 1800 nm.

Another aspect of the invention includes crystalline composition. Thecrystalline composition may comprises any of the compositions describedherein and have a molecular packing arranging defined by a triclinic,space group P1⁻(no. 2) or a orthorhombic, space group Pna21 (no. 33). Inone embodiment, the composition has a molecular packing arrangingdefined by the triclinic, space group P1­⁻(no. 2) and lattice parametersof a= 14.1 ± 0.1 Å, b = 15.3 ± 0.1 Å, c = 23.3 ± 0.1 Å, α = 84.6 ± 0.1°,β = 82.2 ± 0.1°, and γ = 66.5 ± 0.1°. In another embodiment, thecomposition has a molecular packing arranging defined by theorthorhombic, space group Pna2₁ (no. 33) and lattice parameters of a =27.2 ± 0.1 Å, b = 20.4 ± 0.1 Å, and c = 16.7 ± 0.1 Å. Suitably, thecomposition may comprise six counter anions for every [2]catenane.

Near infrared dyes, memory devices, or energy storage materials may beprepared from any of the compositions described herein.

Yet another aspect of the invention is methods for preparing[2]catenanes. The method may comprise contacting a cationic ring with acationic guest molecule in the presence of reducing agents, i.e., Cudust, Zu dust, or CoCp2, etc., thereby reducing the cationic ring andthe cationic guest molecule and forming a radical cationic inclusioncomplex and reacting the guest molecule of the radical cationicinclusion complex with a ring-closing reagent to prepare the [2]catenaneor reaching the termini of the guest molecule of the radical cationicinclusion complex with each other to prepare the [2]catenane. In someembodiments, the method further comprises reducing the [2]catenane withreducing agent to prepare a reduced [2]catenane. In some embodiments,the cationic ring is mCBPQT⁴⁺. In one embodiment, the cationic guestmolecule may be

In another embodiment, the cationic guest molecule may be

Suitably, the ring-closing reagent may be 4,4′-bipyridine.

These and other aspects of the invention will be described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting embodiments of the present invention will be described byway of example with reference to the accompanying figures, which areschematic and are not intended to be drawn to scale. In the figures,each identical or nearly identical component illustrated is typicallyrepresented by a single numeral. For purposes of clarity, not everycomponent is labeled in every figure, nor is every component of eachembodiment of the invention shown where illustration is not necessary toallow those of ordinary skill in the art to understand the invention.

FIGS. 1A-1C. FIG. 1A) Structural formulas and cavity sizes of CBPQT⁴⁺and mCBPQT⁴⁺. FIG. 1B) The radical host-guest pairing interactionsbetween mCBPQT^(2(•+)) and the dimethyl viologen radical cation, and thecorresponding association constant (K_(a)) in MeCN. FIG. 1C) Thereduction potentials, radical stability, and corresponding referenceliterature of different viologen derivatives-including the newlydesigned [2]catenanes mHe[2]C⁸⁺ and mHo[2]C⁸⁺— indicating the positivecorrelation between the reduction potential, and stability of theradicals.

FIGS. 2A-2F. Solid-state structures. FIG. 2A) A side-on view showing thedihedral angle between the BIPY units in mHe[2]C^(2•6+). FIG. 2B) Atop-down view showing the distances and the torsion angles betweenstacked units in mHe[2]C^(2•6+). FIG. 2C) A side-on view showing thatthere are six PF₆ ⁻ anions surrounding every mHe[2]C^(2•6+). FIG. 2D) Aside-on view showing the dihedral angle between the BIPY units inmHo[2]C^(2•6+). FIG. 2E) A top-down view showing the distances and thetorsion angles between stacked units in mHo[2]C^(2•6+). FIG. 2F) Aside-on view showing that there are six PF₆ ⁻ anions surrounding everymHo[2]C^(2•6+).

FIG. 3 . Cyclic voltammograms of mHe[2]C·6PF₆ (0.50 mM) and mHo[2]C·6PF₆(0.50 mM) with the redox potentials marked on all peaks.

FIGS. 4A-4D. Vis/NIR Absorption spectra of the different redox statesobtained employing electrochemical reduction at different voltages. FIG.4A) mHe[2]C^(4(•+)) (-0.50 V) (maxima indicated at 1070 nm);mHe[2]C^(2•6+) (+0.10 V) (maxima indicated at 1450 nm); mHe[2]C^(•7+)(+0.42 V) (maxima indicated at 1800 nm). FIG. 4B) mHo[2]C^(4(•+)) (-0.50V) (maxima indicated at 1070 nm); mHo[2]C^(2•6+) (+0.10 V) (maximaindicated at 1480 nm); mHo[2]C^(•7+) (+0.50 V) (maxima indicated at 1750nm). Reference electrode: Ag/AgCl. FIG. 4C) EPR spectra of mHe[2]C^(•7+)(humped line) and mHe[2]C^(2•6+) (flat line). FIG. 4D) EPR spectra ofmHo[2]C^(•7+) (humped line) and mHo[2]C^(2•6+) (flat line).

FIGS. 5A-5D. Spin-density distribution of: FIG. 5A) mHe[2]C^(•7+); FIG.5B) mHe[2]C^(2•6+); FIG. 5C) mHo[2]C^(•7+); FIG. 5D) mHo[2] C^(2•6+),showing the spin densities are located in the two inner BIPY units forthe two [2]catenanes in both mono- and bisradical states.

FIG. 6 . Time-dependent Vis/NIR absorption spectra in 30 min intervals(MeCN, optical length: 2 mm) of mHo[2]C^(4(•+)) upon exposing underambient conditions for 16 h.

FIG. 7 . Time-dependent Vis/NIR absorption spectra in 30 min intervals(MeCN, optical length: 2 mm) of mHo[2]C^(4(•+)) upon exposing underambient conditions for 16 h.

FIG. 8 . Time-dependent Vis/NIR absorption spectra (10 mM, MeCN, opticallength: 2 mm) of mHe[2]C^(2•6+) upon exposing under ambient conditions.

FIG. 9 . Time-dependent Vis/NIR absorption spectra (10 mM, MeCN, opticallength: 2 mm) of mHo[2]C^(2•6+) upon exposing under ambient conditions.

DETAILED DESCRIPTION OF THE INVENTION

Disclosed herein are [2]catenane compositions that allow for air-stableorganic radicals. The protection afforded the [2]catenanes by mechanicalbonds allow for the remarkable stability of these radicals.

Catenanes are organic compounds having two or more macrocyclic ringsconnected in the manner of links in a chain, without a covalent bond.Macrocycles are a cyclic macromolecular or a macromolecular cyclicportion of a macromolecule. A molecule of high relative molecular mass,the structure of which essentially comprises the multiple repetition ofunits derived, actually or conceptually, from molecules of low relativemolecular mass. As demonstrated in the Examples that follow, thecatenanes are [2]catenanes having two mechanically interlocked ringsthat result in air-stable radicals, such as bis- and mono-radicals.Suitably, an “air-stable” radical is a radical stable in air for atleast 1 hour, 1 day, 1 week, or more. In some embodiments, theair-stable radical is a radical in a bisradical hexacationic state, amonoradical heptacationic state, or a mixture of both states.

The first ring and the second ring of the catenane each comprise analternating cyclic arrangement of unsubstituted or substituted4,4′-bipyridinium (BIPY) and phenylene subunits. An exemplary BIPYsubunit is a subunit of Formula I,

As disclosed in the examples that follow, the BIPY subunit comprisesunsubstituted pyridine groups. Derivatives of the unsubstituted BIPYsubunit may be prepared and used to form the catenane compositionsdescribed herein by replacing any of the hydrogens on either or both ofthe pyridine rings with one or more substituents. Exemplary substituentsR¹ and R² include, but are not limited to, —CH₃, —OH, —NH₂, —SH, —CN,—NO₂, —F, —Cl, —Br, —I moieties. R¹ and R² may be independentlyselected. In some instances, R¹ and R² are the same. In other instances,R¹ and R² are the different. Because BIPY subunits are threaded throughthe opposite macrocyclic ring, the substituents on a threaded BIPYsubunit must be small enough to allow threading. The second ring mayfurther comprise an additional BIPY subunit that is not threaded throughthe macrocycle of the opposite ring. Because the additional BIPY subunitis not threaded, substituents such as C₁-C₁₂ alkyl, C₂-C₁₂ alkenyl,C₂-C₁₂ alkynyl, C₁-C₁₂ carboxy, C₁-C₁₂ carbonyl, C₁-C₁₂ aldehyde, orC₁-C₁₂ alkoxy moieties having too much steric bulk to allow threadingmay also be used for this subunit. In some embodiments, the substituentscomprise C₁-C₆ alkyl, C₂-C₆ alkenyl, C₂-C₆ alkynyl, C₁-C₆ carboxy, C₁-C₆carbonyl, C₁-C₆ aldehyde, or C₁-C₆ alkoxy moieties or C₁-C₄ alkyl, C₂-C₄alkenyl, C₂-C₄ alkynyl, C₁-C₄ carboxy, C₁-C₄ carbonyl, C₁-C₄ aldehyde,or C₁-C₄ alkoxy moieties.

When a ring comprises two BIPY subunits, the BIPY subunits can be thesame or different. Both BIPY units may be unsubstituted, one BIPYsubunit may be unsubstituted and the other substituted, or both BIPYsubunits may be substituted. When both BIPY subunits are substituted,the BIPY subunits may comprise the same or different substituents.

The BIPY subunits may access a number of different redox states,including as a BIPY²⁺ dication or as a BIPY^(•+) radical cation. FormulaI may represent the BIPY²⁺, BIPY^(•+), or BIPY⁰ redox state depending oncontext. Moreover, when a ring comprises two BIPY subunits, the subunitsmay be in the same redox state or different redox states.4,4′-Bipyridinium radical cations (BIPY^(•+)) tend to form (BIPY^(•+))₂dimers in a ‘face-to-face’ manner in the solid state as a result offavorable radical-pairing interactions. Conversely, in a dilutesolution, (BIPY^(•+))₂ dimers are prone to dissociate because of theirlow association constants.

The BIPY subunits are linked by phenylene subunits, such asmeta-phenylene and/or para-phenylene subunits that may optionally haveone or more linkers for joining the BIPY subunits to the phenylenesubunits. As disclosed in the examples that follow, the phenylenesubunits are unsubstituted. Derivatives of the unsubstitutedpara-phenylene and/or meta-phenylene subunit may be prepared and used toform the catenane compositions described herein by replacing any of thehydrogens on the phenylene with one or more substituents. Because thephenylene subunits may be threaded through the opposite macrocyclic ringto prepare the catenane, the substituents on a threaded phenylenesubunit must be small enough to allow threading. Exemplary substituentsinclude, but are not limited to, alkyl, alkenyl, alkynyl, carboxy,carbonyl, aldehyde, alkoxy, —OH, —NH₂, —SH, —CN, —NO₂, —N₃, —F, —Cl,—Br, —I moieties. In some embodiments, the substituent is -R,-OR, —NH₂,-COOR, —CN, —N₃, —CH═CHR, —NO₂, —F, —Cl, —Br, —I moieties where R is analkyl, such as a C₁-C₁₂ alkyl, C₁-C₆ alkyl, or C₁-C₄ alkyl.

In some embodiments, the BIPY subunits are linked through apara-xylylene and/or a meta-xylylene subunits where the methylenes ofthe xylylene are linkers for joining the BIPY subunits to the phenylenesubunits. The xylylene subunits may be substituted. Exemplarysubstituents for the xylylene subunit include, but are not limited to,alkyl, alkenyl, alkynyl, carboxy, carbonyl, aldehyde, alkoxy, —OH, —NH₂,—SH, —CN, —NO₂, —F, —Cl, —Br, —I moieties. In some embodiments, thesubstituent is -R,-OR, —NH₂, -COOR, —CN, —N₃, —CH═CHR, —NO₂, —F, —Cl,—Br, —I moieties where R is an alkyl, such as a C₁-C₁₂ alkyl, C₁-C₆alkyl, or C₁-C₄ alkyl.

An exemplary embodiment of one or both of the rings of the [2]catenaneis mCBPQT,

Derivatives may also be prepared by substituting any of the hydrogens onany of the aromatic rings of the BIPY subunit as described above. The m-and p-xylylene subunits may be similarly substituted as described above.

Another exemplary embodiment of one of the rings of the [2]catenane isCBPQT,

Derivatives may also be prepared by substituting any of the hydrogens onany of the aromatic rings of the BIPY subunit as described above. Thep-xylylene subunits may be similarly substituted as described above.

In an embodiment of the invention, the [2]catenane composition comprisestwo mechanically interlocked mCBPQT. This [2]catenane may be referred toas mHo[2]C.

In an embodiment of the invention, the [2]catenane composition comprisesone mechanically interlocked mCBPQT and one mechanically interlockedCBPQT. This [2]catenane may be referred to as mHe[2]C.

The mechanically interlocked compositions may be prepared by providing aradical cationic inclusion complex and preforming a ring-closingreaction. The present methods uses Cu dust to prepare the inclusioncomplexes, but other reducing agents may also be used. Prior methods forpreparing inclusion complexes used Zn dust, however Zn dust must beremoved before performing the ring-closing reaction as will over-reduceradical cation, preparing a neutral state, if it remains in the reactionmixture. The advantage of using Cu dust is that it can remain in thereaction mixture with no threat of over-reduction, and can reducecontinuously the newly formed radical cations.

The method for preparing a [2]catenane may comprise contracting acationic ring with a cationic guest molecule in the presence of reducingagent, such as Cu dust. The presence of the Cu dust reduces the cationicring and the cationic guest molecule into radical cations, respectively.In an embodiment, the cationic ring is mCBPQT⁴⁺ but any of the ringsdescribed herein may be employed.

Scheme 1 illustrates two embodiments for preparing the [2]catenane. In afirst embodiments, the guest molecule of the radical cationic inclusioncomplex is reacted with a ring-closing reagent to prepare the[2]catenane. In an alternative embodiment, the termini of the guestmolecule of the radical cationic inclusion complex may be reacted witheach other to prepare the [2]catenane.

Scheme 1. Synthetic route for the preparation of mHe[2]C·6PF₆,mHo[2]C·6PF₆, mHe[2]C·8PF₆ and mHo[2]C·8PF₆.

As evidenced by two highly charged [2]catenanes, air-stable singletbisradicals may be prepared and isolated. mHe[2]C•6PF₆ and mHo[2]C•6PF₆that were synthesized by exploiting radical host-guest templationbetween BIPY^(•+) derivatives and mCBPQT^(2(•+)). In contrast to other[2]catenanes that have been isolated as air-stable monoradicals, bothmHe[2]C•6PF₆ and mHo[2]C•6PF₆, exist as air-stable singlet bisradicalsas evidenced by both X-ray crystallography in the solid state and EPRspectroscopy in solution. Electrochemical studies indicate that thefirst two reduction peaks of these two [2]catenanes are shiftedsignificantly to more positive potentials, a feature which isresponsible for their extraordinary stability in air. The mixed-valencenature of the mono- and bisradical states endows them with uniqueNIR-absorption properties, e.g., NIR absorption bands for the mono- andbisradical states observed at ∼1800 and ∼1450 nm, respectively. These[2]catenanes are useful in applications that include NIR photothermalconversion, UV/Vis/NIR multiple-state electrochromic materials, andmultiple-state memory devices. Our findings highlight the principle of“mechanical-bond-induced-stabilization” as an efficient strategy fordesigning persistent organic radicals.

N,N′-Disubstituted-4,4′-bipyridinium dications (BIPY²⁺), also known asviologens¹⁰, are electron acceptors that can undergo two sequential andreversible one-electron reductions with half-wave potentials of -0.30and -0.71 V (versus Ag/AgCl in MeCN). The bipyridinium radical cation(BIPY^(•+)), which is generated from the one-electron reduction ofBIPY²⁺, is a well-known thermally stable radical species in an inertatmosphere, and can undergo (noncovalent) π-dimerization¹¹ on account ofradical-radical interactions; such interactions have been exploitedintensively in supramolecular chemistry¹² and mechanostereochemistry¹³.Although, BIPY^(•+) cannot undergo σ-dimerization to form a covalentbond, it is unstable when exposed to air because the BIPY²⁺/BIPY^(•+)reduction potential (-0.30 V versus Ag/AgCl) is not sufficientlypositive for the radicals to resist aerobic oxidation. One way to tunethe reduction potential of viologens towards more positive valuesinvolves introducing electron-withdrawing substituents onto viologenderivatives that makes them more electron-deficient, as exemplified(FIGS. 1A-1C) by tetramethyl esters functionalized¹⁴ dimethyl viologen,TEMV²⁺. The first reduction potential of TEMV²⁺ is shifted to around+0.27 V versus Ag/AgCl relative to that (-0.30 V) of the originaldimethyl viologen radical cation (MV^(•+)), and so the air-stability ofthe TEMV^(•+) radical cation turned out to be improved¹⁴ significantly.

Cyclobis(paraquat-p-phenylene) bisradical dication CBPQT^(2(•+)), shownin FIG. 1A, can accommodate a BIPY^(•+) radical cation to form thetrisradical tricationic complex BIPY^(•+)⊂CBPQT^(2(•+)) in MeCN.¹⁵ Usingthis complex as a templating motif, we have synthesized¹⁶ a series ofhighly positively charged mechanically interlocked molecules (MIMs),i.e., Rox-3V⁶⁺ and Ho[2]C⁸⁺ shown in FIG. 1C. We found that thereduction potentials of these MIMs are shifted to significantly morepositive values when the positively charged components in the MIMs areforced into nano-confinement as a result of mechanical bonding whichstabilizes the radical states under ambient conditions. This property isespecially evident in the homo[2]catenane, Ho[2]C⁸⁺, in which the fourrepulsive viologen units are obliged to stack with π-overlap in a verysmall volume (<1.25 nm³), a situation that brings about a strongtendency for Ho[2]C⁸⁺ to accept electrons, resulting in thestabilization of the monoradical Ho[2]C^(•7+) state under ambientconditions.

mCBPQT^(2(•+)) also associates with MV^(•+) in MeCN, despite its cavitybeing significantly smaller (FIGS. 1A and 1B) than that present inCBPQT^(2(•+)). Since the mCBPQT⁴⁺ cavity is smaller¹⁷ than that ofCBPQT⁴⁺, the four electrostatically repulsive viologen units stack in aneven more compact manner than those present in Ho[2]C⁸⁺. A boldeddescriptor may denote a compound, be it free or complexed, and anunbolded descriptor refers to either (i) a component within a moleculeor (ii) a component part of a mechanically interlocked molecule.Consequently, both of the two inner BIPY²⁺ units in mHe[2]C⁸⁺ andmHo[2]C⁸⁺ are expected to be more easily reduced than those in Ho[2]C⁸⁺.If the second reduction potentials of mHe[2]C⁸⁺ and mHo[2]C⁸⁺ areshifted positively to values that make aerobic oxidation difficult, thenthe bisradical forms— namely, mHe[2]C^(2•6+) and mHo[2]C^(2•6+)—will bestable under ambient conditions. Since the first reduction potential ofTEMV²⁺ (FIGS. 1A-1C) is around +0.27 V versus Ag/AgCl, we believe that,if the second reduction potentials of the new [2]catenanes are morepositive than +0.27 V, then their bisradical states will have comparableor even better air-stability than TEMV^(•+). The mixed-valence nature ofthe mono- and bisradical states is responsible for their unique NIRabsorption properties. These new [2]catenanes are useful in a variety ofways, such as in NIR-II photothermal conversion,²² UV/Vis/NIR multistateelectrochromic materials,²³ and multistate information storage/memorydevices²⁴.

Definitions

As used herein, an asterick “*” or a plus sign “+” may be used todesignate the point of attachment for any radical group or substituentgroup.

The term “alkyl” as contemplated herein includes a straight-chain orbranched alkyl radical in all of its isomeric forms, such as a straightor branched group of 1-12, 1-10, or 1-6 carbon atoms, referred to hereinas C1-C12 alkyl, C1-C10-alkyl, and C1-C6-alkyl, respectively.

The term “alkylene” refers to a diradical of an alkyl group. Anexemplary alkylene group is —CH₂CH₂—.

The term “haloalkyl” refers to an alkyl group that is substituted withat least one halogen. For example, —CH₂F, —CHF₂, —CF₃, —CH₂CF₃, —CF₂CF₃,and the like

The term “heteroalkyl” as used herein refers to an “alkyl” group inwhich at least one carbon atom has been replaced with a heteroatom(e.g., an O, N, or S atom). One type of heteroalkyl group is an“alkoxyl” group

The term “alkenyl” as used herein refers to an unsaturated straight orbranched hydrocarbon having at least one carbon-carbon double bond, suchas a straight or branched group of 2-12, 2-10, or 2-6 carbon atoms,referred to herein as C₂-C₁₂-alkenyl, C₂-C₁₀-alkenyl, and C₂-C₆-alkenyl,respectively

The term “alkynyl” as used herein refers to an unsaturated straight orbranched hydrocarbon having at least one carbon-carbon triple bond, suchas a straight or branched group of 2-12, 2-10, or 2-6 carbon atoms,referred to herein as C₂-C₁₂-alkynyl, C₂-C₁₀-alkynyl, and C₂-C₆-alkynyl,respectively

The term “cycloalkyl” refers to a monovalent saturated cyclic, bicyclic,or bridged cyclic (e.g., adamantyl) hydrocarbon group of 3-12, 3-8, 4-8,or 4-6 carbons, referred to herein, e.g., as “C₄₋₈-cycloalkyl,” derivedfrom a cycloalkane. Unless specified otherwise, cycloalkyl groups areoptionally substituted at one or more ring positions with, for example,alkanoyl, alkoxy, alkyl, haloalkyl, alkenyl, alkynyl, amido, amidino,amino, aryl, arylalkyl, azido, carbamate, carbonate, carboxy, cyano,cycloalkyl, ester, ether, formyl, halogen, haloalkyl, heteroaryl,heterocyclyl, hydroxyl, imino, ketone, nitro, phosphate, phosphonato,phosphinato, sulfate, sulfide, sulfonamido, sulfonyl or thiocarbonyl. Incertain embodiments, the cycloalkyl group is not substituted, i.e., itis unsubstituted.

The term “cycloalkylene” refers to a diradical of an cycloalkyl group.

The term “partially unsaturated carbocyclyl” refers to a monovalentcyclic hydrocarbon that contains at least one double bond between ringatoms where at least one ring of the carbocyclyl is not aromatic. Thepartially unsaturated carbocyclyl may be characterized according to thenumber oring carbon atoms. For example, the partially unsaturatedcarbocyclyl may contain 5-14, 5-12, 5-8, or 5-6 ring carbon atoms, andaccordingly be referred to as a 5-14, 5-12, 5-8, or 5-6 memberedpartially unsaturated carbocyclyl, respectively. The partiallyunsaturated carbocyclyl may be in the form of a monocyclic carbocycle,bicyclic carbocycle, tricyclic carbocycle, bridged carbocycle,spirocyclic carbocycle, or other carbocyclic ring system. Exemplarypartially unsaturated carbocyclyl groups include cycloalkenyl groups andbicyclic carbocyclyl groups that are partially unsaturated. Unlessspecified otherwise, partially unsaturated carbocyclyl groups areoptionally substituted at one or more ring positions with, for example,alkanoyl, alkoxy, alkyl, haloalkyl, alkenyl, alkynyl, amido, amidino,amino, aryl, arylalkyl, azido, carbamate, carbonate, carboxy, cyano,cycloalkyl, ester, ether, formyl, halogen, haloalkyl, heteroaryl,heterocyclyl, hydroxyl, imino, ketone, nitro, phosphate, phosphonato,phosphinato, sulfate, sulfide, sulfonamido, sulfonyl or thiocarbonyl. Incertain embodiments, the partially unsaturated carbocyclyl is notsubstituted, i.e., it is unsubstituted.

The term “aryl” is art-recognized and refers to a carbocyclic aromaticgroup. Representative aryl groups include phenyl, naphthyl, anthracenyl,and the like. The term “aryl” includes polycyclic ring systems havingtwo or more carbocyclic rings in which two or more carbons are common totwo adjoining rings (the rings are “fused rings”) wherein at least oneof the rings is aromatic and, e.g., the other ring(s) may becycloalkyls, cycloalkenyls, cycloalkynyls, and/or aryls. Unlessspecified otherwise, the aromatic ring may be substituted at one or morering positions with, for example, halogen, azide, alkyl, aralkyl,alkenyl, alkynyl, cycloalkyl, hydroxyl, alkoxyl, amino, nitro,sulfhydryl, imino, amido, carboxylic acid, -C(O)alkyl, -CO₂alkyl,carbonyl, carboxyl, alkylthio, sulfonyl, sulfonamido, sulfonamide,ketone, aldehyde, ester, heterocyclyl, aryl or heteroaryl moieties,—CF₃, —CN, or the like. In certain embodiments, the aromatic ring issubstituted at one or more ring positions with halogen, alkyl, hydroxyl,or alkoxyl. In certain other embodiments, the aromatic ring is notsubstituted, i.e., it is unsubstituted. In certain embodiments, the arylgroup is a 6-10 membered ring structure.

The terms “heterocyclyl” and “heterocyclic group” are art-recognized andrefer to saturated, partially unsaturated, or aromatic 3- to 10-memberedring structures, alternatively 3-to 7-membered rings, whose ringstructures include one to four heteroatoms, such as nitrogen, oxygen,and sulfur. The number of ring atoms in the heterocyclyl group can bespecified using 5 Cx-Cx nomenclature where x is an integer specifyingthe number of ring atoms. For example, a C₃-C₇ heterocyclyl group refersto a saturated or partially unsaturated 3- to 7-membered ring structurecontaining one to four heteroatoms, such as nitrogen, oxygen, andsulfur. The designation “C₃-C₇” indicates that the heterocyclic ringcontains a total of from 3 to 7 ring atoms, inclusive of any heteroatomsthat occupy a ring atom position.

The terms “amine” and “amino” are art-recognized and refer to bothunsubstituted and substituted amines, wherein substituents may include,for example, alkyl, cycloalkyl, heterocyclyl, alkenyl, and aryl.

The terms “alkoxyl” or “alkoxy” are art-recognized and refer to an alkylgroup, as defined above, having an oxygen radical attached thereto.Representative alkoxyl groups include methoxy, ethoxy, tert-butoxy andthe like.

An “ether” is two hydrocarbons covalently linked by an oxygen.Accordingly, the substituent of an alkyl that renders that alkyl anether is or resembles an alkoxyl, such as may be represented by one of—O—alkyl, —O—alkenyl, —O—alkynyl, and the like.

An “epoxide” is a cyclic ether with a three-atom ring typically includetwo carbon atoms and whose shape approximates an isosceles triangle.Epoxides can be formed by oxidation of a double bound where the carbonatoms of the double bond form an epoxide with an oxygen atom.

The term “carbonyl” as used herein refers to the radical —C(O)—.

The term “carboxamido” as used herein refers to the radical —C(O)NRR′,where R and R′ may be the same or different. Rand R′ may beindependently alkyl, aryl, arylalkyl, cycloalkyl, formyl, haloalkyl,heteroaryl, or heterocyclyl.

The term “carboxy” as used herein refers to the radical -COOH or itscorresponding salts, e.g. —COONa, etc.

The term “amide” or “amido” as used herein refers to a radical of theform —R¹C(O)N(R²)—, —R¹C(O)N(R²)R³—, —C(O)NR²R³, or —C(O)NH₂, whereinR¹, R² and R³ are each independently alkoxy, alkyl, alkenyl, alkynyl,amide, amino, aryl, arylalkyl, carbamate, cycloalkyl, ester, ether,formyl, halogen, haloalkyl, heteroaryl, heterocyclyl, hydrogen,hydroxyl, ketone, or nitro.

Miscellaneous

Unless otherwise specified or indicated by context, the terms “a”, “an”,and “the” mean “one or more.” For example, “a molecule” should beinterpreted to mean “one or more molecules.”

As used herein, “about”, “approximately,” “substantially,” and“significantly” will be understood by persons of ordinary skill in theart and will vary to some extent on the context in which they are used.If there are uses of the term which are not clear to persons of ordinaryskill in the art given the context in which it is used, “about” and“approximately” will mean plus or minus ≤10% of the particular term and“substantially” and “significantly” will mean plus or minus >10% of theparticular term.

As used herein, the terms “include” and “including” have the samemeaning as the terms “comprise” and “comprising.” The terms “comprise”and “comprising” should be interpreted as being “open” transitionalterms that permit the inclusion of additional components further tothose components recited in the claims. The terms “consist” and“consisting of” should be interpreted as being “closed” transitionalterms that do not permit the inclusion additional components other thanthe components recited in the claims. The term “consisting essentiallyof” should be interpreted to be partially closed and allowing theinclusion only of additional components that do not fundamentally alterthe nature of the claimed subject matter.

All methods described herein can be performed in any suitable orderunless otherwise indicated herein or otherwise clearly contradicted bycontext. The use of any and all examples, or exemplary language (e.g.,“such as”) provided herein, is intended merely to better illuminate theinvention and does not pose a limitation on the scope of the inventionunless otherwise claimed. No language in the specification should beconstrued as indicating any non-claimed element as essential to thepractice of the invention.

All references, including publications, patent applications, andpatents, cited herein are hereby incorporated by reference to the sameextent as if each reference were individually and specifically indicatedto be incorporated by reference and were set forth in its entiretyherein.

Preferred aspects of this invention are described herein, including thebest mode known to the inventors for carrying out the invention.Variations of those preferred aspects may become apparent to those ofordinary skill in the art upon reading the foregoing description. Theinventors expect a person having ordinary skill in the art to employsuch variations as appropriate, and the inventors intend for theinvention to be practiced otherwise than as specifically describedherein. Accordingly, this invention includes all modifications andequivalents of the subject matter recited in the claims appended heretoas permitted by applicable law. Moreover, any combination of theabove-described elements in all possible variations thereof isencompassed by the invention unless otherwise indicated herein orotherwise clearly contradicted by context.

EXAMPLES General Methods

All reagents were purchased from commercial suppliers and used withoutfurther purification. Compounds 1•2PF₆, CBPQT•4PF₆, mCBPQT•4PF₆ wereprepared^(S1) according to literature procedures. Thin layerchromatography (TLC) was performed on silica gel 60 F254 (E. Merck).Column chromatography was carried out on silica gel 60F (Merck 9385,0.040-0.063 mm). C-18 Columns were used for analytical and preparativereverse-phase high-performance liquid-chromatography (RP-HPLC) onAgilent 1260 infinity LC equipped with Agilent 6120 LC/MS electrospraysystem and Shimadzu Prominence LC-8a instruments, respectively, elutedwith H₂O/MeCN (0.1 % v/v TFA) and monitored using a UV detector (λ = 360nm). UV/Vis Spectra were recorded at room temperature on a ShimadzuUV-3600 spectrophotometer. Nuclear magnetic resonance (NMR) spectra arerecorded on Agilent DD2 500 as well as on Bruker Avance III 400 and 500spectrometers, with working frequencies of 400 and 500 MHz for ¹H, aswell as 100 and 125 MHz for ¹³ C nuclei, respectively. Chemical shiftswere reported in ppm relative to the signals corresponding to theresidual non-deuterated solvents (CD₃CN: δ_(H) =1.94 ppm and δ_(C) =118.26 ppm for ¹³CN). High-resolution mass spectra (HR-ESI) weremeasured on a Finnigan LCQ iontrap mass spectrometer. Electronparamagnetic resonance (EPR) measurements at X-band (9.5 GHz) wereperformed with a Bruker Elexsys E580, equipped with a 4122SHQEresonator. All samples were prepared in an Argon-filled atmosphere.Scans were performed with magnetic field modulation amplitude of 1 G andnon-saturating microwave power between 0.4 and 0.6 mW. Samples werecontained in quartz tubes with I.D. 1.50 mm and O.D. 1.80 mm and sealedwith a clear ridged UV curin epoxy (IllumaBond 60-7160RCL) and usedimmediately after preparation. Cyclic voltammetry (CV) and differentialpulse voltammetry (DPV) experiments were carried out at room temperaturein Argon-purged MeCN solutions with a Gamry Multipurpose instrument(Reference 600) interfaced to a PC. CV Experiments were performed usinga glassy carbon working electrode (0.071 cm²). The electrode surface waspolished routinely with 0.05 µm alumina-water slurry on a felt surfaceimmediately before use. The counter electrode was a Pt coil and thereference electrode was Ag/AgCl electrode. The concentration of thesupporting electrolyte tetrabutylammonium hexafluorophosphate (NH₄PF₆)was 0.1 M.

Synthesis

The highly stable bisradical [2]catenanes, mHe[2]C•6PF₆ andmHo[2]C·6PF₆, were synthesized by modifying the previously reportedprocedure^(16b) for the preparation of Ho[2]C•7PF₆ The mCBPQT•4PF₆ hostand the guest molecule 1•2PF₆ were reduced (Scheme 1) with an excess ofCu dust in MeCN in a N₂-filled glovebox for 2 h, producing thetrisradical tricationic inclusion complex 1^(•+)⊂mCBPQT^(2(•+)).4,4′-Bipyridine was then added to this solution so as to react with1•2PF₆ and give mHe[2]C^(3•5+) as the ring-closure product, which wasthen reduced again by the Cu dust to give mHe[2]C^(4(•+)). The reactionmixture was stirred at room temperature under N₂ for 1 week, after whichit was exposed to air. Purification by reverse phase columnchromatography, followed by counterion exchange, and recrystallization(see Section B) afforded mHe[2]C•6PF₆ in 30% yield.

In a similar manner, mHo[2]C•6PF₆ was obtained in 19% yield by reactingmCBPQT•4PF₆ with 2•3PF₆ using the same protocol. The lower yield ofmHo[2]C•6PF₆ can be attributed to the smaller cavity of mCBPQT^(2(+•))compared to that of CBPQT^(2(+•)), which renders the ring-closure stepfor mHo[2]C•6PF₆ more difficult than that for mHe[2]C•6PF₆. Highresolution electrospray ionization mass spectrometry (ESI-MS) confirmedthat both catenanes possess the same molecular formula, i.e.,C₇₂H₆₄F₃₆N₈P₆.

¹H NMR Spectra have been recorded for both catenanes in their fullyoxidized states— namely, mHe[2]C·8PF₆ and mHo[2]C·8PF₆ —which wereobtained by oxidizing the as-synthesized catenanes with an excess ofNOPF₆. Because of their lower symmetries, both mHe[2]C·8PF₆ andmHo[2]C·8PF₆ display much more complicated ¹H NMR spectra than thatobserved for Ho[2]C·8PF₆. The characteristic signals of these[2]catenanes correspond to the proton resonances of the innermost BIPY²⁺units, which are strongly shielded and consequently shifted dramaticallyupfield, into the 4-5 ppm region. Notably, the eight resonances for theinnermost protons are separated into two sets of signals formHe[2]C·8PF₆ (two protons resonate at ∼5.10 ppm and six protons resonateat ∼4.25 ppm), while these same eight proton resonances in the spectrumof mHo[2]C·8PF₆ are separated into four sets of signals at 5.29, 4.98,4.38, and 4.07 ppm. These observations can be attributed to theasymmetric cavities of the mCBPQT⁴⁺ component ring(s). The encapsulatedBIPY²⁺ unit(s) are obliged to reside closer to the p-xylylene linker endthan the m-xylylene linker end in order to attenuate Coulombicrepulsions as much as possible. As a consequence, the innermost protonson the BIPY²⁺ units of mCBPQT⁴⁺ experience different extents ofshielding, leading to well separated chemical shifts. The remainingproton resonances in the spectra of mHe[2]C·8PF₆ and mHo[2]C·8PF₆ arealso more complicated for similar reasons.

The previously reported protocol in Reference 16b uses Zn dust as thereducing agent to generate rapidly the trisradical tricationiccomplexes. Zn dust, however, must be removed from the reaction mixturebefore BIPY is added to the solution so as to react with theencapsulated xylylene dibromide. Zn will over-reduce the viologenradical cation to give its neutral state if it remains in the reactionmixture. Once the substitution is over, however, the newly formed BIPY²⁺cannot be reduced because of the absence of reducing reagents in thesolution. Hence, there will be fast electron exchange between the newlyformed BIPY²⁺ units and the trisradical tricationic complexes, asituation which will reduce the formation of the trisradical tricationiccomplexes and therefore decrease the catenation yield. The advantage ofusing Cu dust is that it can remain in the reaction mixture with nothreat of over-reduction, and can reduce continuously the newly formedBIPY²⁺ units to radical cations. Synthetic Protocols

mHeC•6PF₆: mCBPQT•4PF₆ (330 mg, 0.30 mmol) and 1•2PF₆ (270 mg, 0.34mmol) were dissolved in degassed MeCN (60 mL) in a 100-mL round-bottomedflask in a N₂-filled glovebox. An excess of Cu dust (∼100 mg) was addedwith stirring to this solution. After 2 h, the solution turned fromcolorless to a deep purple color, an observation which is indicative ofthe formation of the trisradical tricationic complex. Then4,4′-bipyridine (54 mg, 0.34 mmol) was added to it, and the resultingmixture was allowed to stand for 7 days at room temperature. Cu Dust wasremoved by filtration and the solvent was evaporated off under vacuum.The crude product was purified using reversed-phase flash chromatography(C₁₈: H₂O/MeCN 0.1% TFA 0-100%), followed using anion exchange from TFA⁻to PF₆ ⁻ by treating the aqueous fractions with an excess of NH₄PF₆,resulting in a white precipitate which was collected by centrifugationand washed with H₂O several times before being dried in vacuo to affordmHe[2]C•6PF₆ as a dark purple solid (170 mg, 30%). For NMR spectroscopiccharacterization, mHe[2]C•6PF₆ (2 mg) was oxidized to mHe[2]C•8PF₆ bythe addition of an excess (1 mg) of NO•PF₆. ¹H NMR (500 MHz, CD₃CN, 298K) of mHe[2]C•8PF₆: δ 9.03 (d, J= 6.6, 6 H), 8.99 (d, J= 7.0 Hz, 2H),8.87 (d, J= 6.6 Hz, 2 H), 8.83 (d, J= 6.6 Hz, 2 H), 8.72 (d, J= 6.0 Hz,2 H), 8.40 (s, br, 1 H), 8.34-8.32 (m, 4 H), 8.17 (d, J = 7.0 Hz, 2 H),8.14 (s, br, 9 H), 8.06 (s, br, 2 H), 7.85 (d, J = 6.5 Hz, 2 H), 7.79(d, J = 6.5 Hz, 4 H), 7.74 (d, J = 6.5 Hz, 2 H), 6.17-6.13 (m, 6 H),6.00-5.96 (m, 10H), 5.09 (s, br, 2 H), 4.28-4.22 (m, 6 H). ¹³ C NMR (125MHz, CD₃CN, 298 K) of mHe[2]C•8PF₆: δ 149.0, 148.8, 148.3, 148.0, 147.7,147.3, 146.9, 146.6, 143.4, 143.2, 142.8, 140.4, 140.0, 138.8, 138.2,136.9, 136.8, 135.8, 135.1, 134.7, 133.0, 132.9, 132.5, 132.0, 128.2,128.0, 127.7, 122.9, 122.9, 122.5, 67.5, 67.2, 67.0, 65.6, 65.5, 65.4.ESI-HRMS for mHe[2]C•6PF₆; Calcd for C₇₂H₆₄F₃₆N₈P₆: m/z = 810.1905 [M-2PF₆]²⁺; found: 810.1889.

4•3PF₆: 3•2PF₆ (350 mg, 0.50 mmol) and (4-(bromomethyl)phenyl)methanol(120 mg, 0.60 mmol) were dissolved in degassed MeCN (60 mL) in a 100-mLround-bottomed flask. The solution was heated to 80° C. for 16 h withstirring. After cooling down to room temp, excess of TBAC1 was added tothe solution. The solids were collected by filtration, and the crudeproduct was purified by reversed-phase flash chromatography (C₁₈:H₂O/MeCN 0.1% TFA 0-100%), followed by anion exchange from TFA⁻ to PF₆ ⁻by treating the aqueous fractions with an excess of NH₄PF₆, resulting ina white precipitate which was collected by centrifugation and washedwith H₂O several times before being dried in vacuo to afford 4•3PF₆ asan off-white solid (211 mg, 45%). ¹H NMR (500 MHz, CD₃CN, 298 K): δ9.00-8.94 (m, 8 H), 8.42-8.37 (m, 8 H), 7.63-7.59 (m, 4 H), 7.59 (s, 4H), 5.87-5.85 (m, 4 H), 5.83 (s, 2 H), 4.65 (s, 2 H). ¹³ C NMR (125 MHz,CD₃CN, 298 K) δ 151.9, 151.4, 151.0, 146.5, 146.4, 145.2, 143.9, 134.7,132.0, 131.6, 131.6, 131.3, 130.2, 128.44, 128.42, 128.36, 128.32,127.4, 100.8, 65.4, 64.9, 64.8.

2•3PF₆: 4•2PF₆ (160 mg, 0.83 mmol) was dissolved in HBr (33% in HOAc)(10 mL) in a 25-mL vial. The solution stirred under rt for 16 before theall the solvents were removed in vacuo. The residue was dissolved inwater (10 mL), and excess NH₄PF₆ were added to the solution. Theprecipitate was collected by filtration, washed with H₂O for severaltimes before being dried in vacuo to afford 2•3PF₆ as an off-white solid(153 mg, 92%). ¹H NMR (500 MHz, CD₃CN, 298 K): δ 9.00-8.93 (m, 8 H),8.41-8.38 (m, 8 H), 7.64-7.50 (m, 8 H), 5.85-5.84 (m, 6 H), 4.654 (s, 2H). ¹³ C NMR (125 MHz, CD₃CN, 298 K) δ 151.9, 151.3, 151.1, 146.5,146.5, 145.2, 143.9, 134.7, 132.0, 131.6, 131.6, 131.3, 130.2, 128.46,128.40, 128.35, 128.2, 127.4, 100.8, 65.4, 65.0, 63.6.

mHoC•6PF₆: Following a procedure similar to that described for thesynthesis of mHe [2]C•6PF₆, the reaction of a mixture composed ofmCBPQT•4PF₆ (220 mg, 0.20 mmol), 2•3PF₆ (142 mg, 0.17 mmol) affordedmHo[2]C•6PF₆ as a dark purple solid (69 mg, 18%). For NMR spectroscopiccharacterization, mHo[2]C•6PF₆ (2 mg) was oxidized to mHo[2]C•8PF₆ bythe addition of an excess (1 mg) of NO•PF₆. ¹H NMR (500 MHz, CD₃CN, 298K) of mHo[2]C·8PF₆: δ 9.09 (d, J= 8.0 Hz, 2 H), 9.03-9.01 (m, 4 H), 8.98(d, J= 7.5 Hz, 2 H), 8.82 (d, J= 6.5 Hz, 2 H), 8.76 (d, J= 6.5 Hz, 8 H),8.42-8.32 (m, 18 H), 7.87-7.85 (m, 4 H), 7.77-7.75 (m, 4 H), 7.65-7.63(m, 2 H), 6.20-5.97 (m, 16 H), 5.29 (s, br, 2 H), 4.98 (s, br, 2 H),4.38 (s, br, 2 H), 4.07 (s, br, 2 H). ¹³C NMR (125 MHz, CD₃CN, 298 K) ofSC•8PF₆: δ 148.3, 147.7, 147.5, 147.0, 146.6, 146.3, 143.3, 142.4,142.2, 140.0, 138.6, 136.6, 136.3, 135.2, 134.5, 132.8, 131.9, 131.4,131.0, 128.1, 127.9, 127.6, 127.3, 67.1, 66.4, 65.4, 65.3. ESI-HRMS formHo[2]C•6PF₆; Calcd for C₇₂H₆₄F₃₆N₈P₆: m/z = 810.1905 [M- 2PF₆]²⁺;found: 810.1895.

X-Ray Crystallography

Single crystals of the two catenanes were grown under ambient conditionsby slowly evaporating Et₂O into 1.0 mM MeCN solutions over a week whichaffords dark red crystals suitable for X-ray crystallographic analysis.The solid-state structures show (FIGS. 2A-2F) that both compoundscrystallize with six PF₆ ⁻ counterions around the catenanes, anobservation which confirms their bisradical hexacationic states underambient conditions. The torsional angles (FIG. 2B) of 20° and 23° forthe A and D units, respectively, in mHe[2]C^(2•6+) are typical ofdicationic BIPY²⁺ units, and indicate that the unpaired electrons arenot located on the A and D units. By contrast, units B and C show (FIG.2B) much smaller torsion angles-8° for unit B and 6° for unit C—indicating that the unpaired electrons are most likely to be locatedbetween these units. Moreover, the centroid-to-centroid distance (3.18Å) between units B and C is significantly shorter than that observedbetween (3.57 Å) units A and B or between (3.53 Å) units C and D. Thevalue of the distance (3.18 Å) between units B and C is a typicallyassociated with radical-radical interactions, an observation thatsupports the existence of radical-radical pairing interactions betweenunits B and C. In the case of mHo[2]C^(2•6+), however, all four units(A, B, C, and D) present (FIG. 2E) near planar conformations withsomewhat smaller (<13°) torsion angles, as well as shorter (<3.57 Å)distances between adjacent units when compared to those inmHe[2]C^(2•6+). These observations can be explained by the smallcavities of the two mCBPQT rings, which force the four viologen units tostack more tightly in mHo[2]C^(2•6+) than in mHe[2]C^(2•6+), therebyleading to flatter viologen-unit conformations and shorter separations.Nevertheless, the torsion angles of the two inner units, B (4°) and C(0°), are still smaller than those of the outer units, A (13°) and D(3°), while the centroid-to-centroid distance (3.12 Å) between units Band C is also smaller than that between (3.39 Å) units A and B orbetween (3.34 Å) units C and D. Therefore, we conclude that the twospins in mHo[2]C^(2•6+) are also mainly located on units B and C. 1)mHe[2]C•6PF₆

-   a) Methods. Single crystals of mHe[2]C•6PF₆ were grown on the    bench-top by slow vapor diffusion of ^(i)Pr₂O into a 1.0 mM solution    in MeCN over the course of a week. A suitable crystal was selected    and mounted in inert oil and transferred to the cold gas stream of a    Bruker Kappa Apex2 diffractometer. The crystal was kept at 100 K    during data collection. Using Olex2^(S2), the structure was solved    with the XM^(S3) structure solution program using dual space and    refined with the XL^(S4) refinement package using least squares    minimization.-   b) Crystal data. Triclinic, space group P1 (no. 2), a= 14.1120(2), b    = 15.2780(2), c = 23.3050(3) Å, α = 84.5780(10), β = 82.1550(10), γ    = 66.5170(10)°, V= 4560.90(11) Å³, Z = 2, T= 100.01(10) K,    µ(CuK_(α)) = 2.200 mm⁻¹, D_(calc)= 1.481 g/mm³, 58617 reflections    measured (3.83 ≤ 2Θ ≤ 146.334), 17719 unique (R_(int) = 0.0557,    R_(sigma) = 0.0565) which were used in all calculations. The final    R₁ was 0.0960 (I > 2σ(I)) and wR₂ was 0.3126 (all data).-   c) Refinement details. The enhanced rigid-bond restraint (SHELX    keyword RIGU) was applied on the disordered PF₆ ⁻ anions. Distance    restraints were also imposed on the disordered anions.-   d) Solvent treatment details. The solvent masking procedure as    implemented in Olex2^(S2) was used to remove the electronic    contribution of solvent molecules from the refinement. As the exact    solvent content is not known, only the atoms used in the refinement    model are reported in the formula here. 2) mHo[2]C•6PF₆-   a) Methods. Single crystals of mHo[2]C•6PF₆ were grown on the    bench-top by slow vapor diffusion of ^(i)Pr₂O into a 1.0 mM solution    in MeCN over the course of a week. A suitable single crystal was    selected and mounted in inert oil and transferred to the cold gas    stream of a Bruker APEX-II CCD diffractometer. The crystal was kept    at 100 K during data collection. Using Olex2^(S2), the structure was    solved with the XM^(S3) structure solution program using dual space    and refined with the XL^(S4) refinement package using least squares    minimization.-   b) Crystal data. Orthorhombic, space group Pna2₁ (no. 33), α =    27.2161(4), b = 20.3618(3), c = 16.7417(3) Å, V = 9277.7(3) Å³, Z=    4, T= 100.01(10) K, µ(CuK_(α)) = 2.163 mm⁻¹, D_(calc) = 1.456 g/mm³,    65212 reflections measured (5.42 ≤ 2Θ ≤ 154.996), 17763 unique    (R_(int) = 0.0527, R_(sigma) = 0.0496) which were used in all    calculations. The final R₁ was 0.0609 (I > 2σ(I)) and wR₂ was 0.1712    (all data).-   c) Refinement details. Distance a restraints were imposed on the    disordered PF₆ ⁻ anions. The enhanced rigid-bond restraint (SHELX    keyword RIGU) was applied to the disordered PF₆ ⁻ anions.-   d) Solvent treatment details. The solvent masking procedure as    implemented in Olex2 was used to remove the electronic contribution    of solvent molecules from the refinement. As the exact solvent    content is not known, only the atoms used in the refinement model    are reported in the formula here. Total solvent accessible volume /    cell = 3555.8 Å³ [32.0%] Total electron count / cell = 429.5

Electrochemistry

Since the stabilities of viologen radicals in air are mainly determinedby their potentials, the redox properties of mHe[2]C·6PF₆ andmHo[2]C·6PF₆ were investigated by cyclic voltammetry (CV). The CV curvefor mHe[2]C·6PF₆ exhibits (FIG. 3 ) five reversible waves correspondingto six discrete accessible redox states, which are similar to thoseobserved previously for Ho[2]C·7PF₆. On the other hand, mHo[2]C·6PF₆displays (FIG. 3 ) six reversible waves because of further splitting ofits last reduction peaks. Notably, the first two reduction peaks of bothmHe[2]C⁸⁺ (E_(red1) = +0.56 V and E_(red2) = +0.29 V versus Ag/AgCl) andmHo[2]C⁸⁺ (E_(red1) = +0.65 V and E_(red2) = +0.34 V versus Ag/AgCl) aresignificantly positively shifted compared to those previously observedfor Ho[2]C⁸⁺ (E_(red1) = +0.34 V and E_(red2) = +0.16 V versus Ag/AgCl).The LUMO energy levels of mHe[2]C⁸⁺ and mHo[2]C⁸⁺ which are calculatedto be -5.17 and -5.27 eV, respectively, both are much lower than thosefor tetracyanoquinodimethane (E_(LUMO) = -4.84 eV) and some other^(2d,8)strong electron acceptors. This observation supports our hypothesis thatdecreasing the cavity sizes of the component-ring in these highlypositively charged catenanes enhances their electron-acceptingabilities. In addition, the last reduction peaks of mHe[2]C⁸⁺ (E_(red5)= -1.31 V versus Ag/AgCl) and mHo[2]C⁸⁺ (E_(red5) = -1.22 V versusAg/AgCl) are both negatively shifted compared to the last reduction peakof Ho[2]C⁸⁺ (E_(red5) = -1.06 V versus Ag/AgCl). Taken all together,these observations demonstrate clearly that subtle differences in thecyclophane linkers influence significantly the redox behavior of thesehighly positively charged [2]catenanes.

Significantly, the potentials of the second reduction peaks of mHe[2]C⁸⁺(E_(red2) = +0.29 V) and mHo[2]C⁸⁺ (E_(red2) = +0.34 V) are comparableto the first reduction potentials of TEMV²⁺ (E_(red1) = +0.29 V, FIG.1C) and Ho[2]C⁸⁺ (E_(red1) = +0.35 V, FIG. 1C). Accordingly, thebisradical states of mHe[2]C⁸⁺ and mHo[2]C⁸⁺ are very like to exhibitsimilar stabilities in air as TEMV^(•+) and Ho[2]C^(•7+).

UV-Vis-NIR Spectroscopy

In order to gain additional insight into their electronic properties, werecorded the UV-Vis-NIR spectra of the two [2]catenanes in their variouselectrochemically generated redox states at different potentials. The CVtraces (FIG. 3 ) reveal that MeCN solutions of mHe[2]C·6PF₆ (+0.80,+0.42, +0.10, and -0.50 V) and mHo[2]C·6PF₆ (+0.80, +0.50, +0.10 and-0.50 V) require different potentials in order to generate thecorresponding redox states (8+, 7+, 6+, and 4+). UV-Vis-NIR Spectra ofthe MeCN solutions of the 7+ (monoradical), 6+ (bisradical), and 4+(tetraradical) redox states were recorded. See FIGS. 4A-4B. Notably, themono- and bisradical states of both [2]catenanes exhibit NIR absorptionbands around 1800 nm and 1440 nm, respectively, and both of theseabsorptions are significantly red-shifted compared^(12c) to those ofBIPY^(•+) (around 600 nm) and the BIPY^(•+) ... BIPY^(•+) supramoleculardimer (800-900 nm). By contrast, the tetracationic tetraradical states,only display NIR bands centered around 1070 nm. These observationsindicate that both the mono- and bisradical states are “mixed-valence”ones. Since all four BIPY units are so closely stacked (distances < 3.6Å), π-overlap and electronic communication exists between all four BIPYunits. Consequently, the unpaired electrons of the radicals are sharedby the four BIPY units to form mixed-valence states with significantlynarrow bandgaps.

We also examined the stabilities of the tetra-, bis-, and monoradicalstates of the two [2]catenanes in air by time-dependent UV-Vis-NIRspectroscopy under ambient conditions. Upon exposure to air, thetetraradicals (mHe[2]C^(4(•+)) and mHo[2]C^(4(•+))) in MeCN wereobserved (FIGS. 6 and 7 ) to decay gradually to their bisradical states(mHe[2]C^(2•6+) and mHo[2]C^(2•6+)) over several hours. The bisradicals(mHe[2]C^(2•6+) and mHo[2]C^(2•6+)), however, exhibit extraordinarystabilities under ambient conditions. MeCN solutions of mHe[2]C^(2•6+)and mHo[2]C^(2•6+) can be stored in air for more than ten days withoutany change (FIGS. 8 and 9 ) in their absorption spectra. Noteworthy isthe fact that, although the monoradicals (mHe[2]C^(•7+) andmHo[2]C^(•7+)) are quite stable in their solid states, in some cases,they tend to be reduced into bisradicals in their solution states whenstored in air for more than 1 weeks.

Organic NIR dyes with absorption bands longer than 1200 nm are notabundant^(3b,21), not only because such red-shifted absorptions aredifficult to achieve, but also because organic compounds with extremelynarrow bandgaps suffer from stability issues. Hence, our results showthat, the mono- and bisradical states of these two [2]catenanes arepromising air-stable NIR-absorbing dyes with significantly red-shiftedabsorption peaks of ca. 1800 and 1450 nm.

The solution of two catenanes with different redox states were preparedby employing electrochemical reductions under different potentials:mHe[2]C•6PF₆ (6.3 mg) was dissolved in MeCN (30 mL) in a N₂-filledglovebox. The solution was then added into the working cell, while theauxiliary electrode chamber was filled with excess of CuPF₆(MeCN)₄dissolved in 0.1 M TBAPF₆/MeCN solution (1 mL). The auxiliary electrodewas made with a platinum wire wrapped with copper wire (diam. 0.25 mm,99.999% trace metals basis from Sigma Aldrich). The whole apparatus wassubjected to different potentials of -0.50, +0.10, and +0.42 V (vsAg/AgCl), respectively. After retaining the each potentials for 10 min,1 mL of each solution was drawn out from the working cell correspondingto +4, +6, and +7 states of mHe[2]C•nPF₆ respectively. Each of the threesolutions was injected into a 2-mm path cuvette which sealed by Tefloncaps and then was analyzed by using Vis/NIR spectroscopy. FormHe[2]C·nPF₆, the +4, +6, and +7 states were obtained using the sameprotocol under the potentials of -0.50, +0.10, and +0.50 V,respectively. The +6 states (bisradical hexacationic states) ofmHo[2]C·nPF₆ and mHe[2]C·nPF₆ were exposed to air for several days andthe Vis/NIR spectra were recorded to test their air-stability.

EPR Spectroscopy

We also recorded (FIGS. 4C-4D) the electron paramagnetic resonance (EPR)spectra of the bisradicals (mHe[2]C^(2·6+) and mHo[2]C^(2·6+)) in MeCN.The very weak signals, which were observed, are almost negligiblecompared with the signal intensities of the monoradicals (FIGS. 4C-4D)recorded under similar conditions. These observations are in accordancewith previously reported^(16b) results, indicating that the unpairedelectrons in the bisradicals are coupled antiferromagnetically and existas ground-state singlets. The relatively weak signals observed in theEPR spectra can be attributed to the thermally populated triplet statesof the bisradicals.

DFT Calculations

DFT calculations were performed in order to probe the electronicproperties of the two catenanes. The results illustrate (FIGS. 5A-5D)that the spin densities in both mono- (mHe[2]C^(·7+) and mHo[2]C^(·7+))and bisradical (mHe[2]C^(2·6+) and mHo[2]C^(2·6+)) states are located ontheir two innermost BIPY^(2+/·+) units in accordance with theexperimental results. The theoretical association energies (Table S1)for the formation of catenanes in their different redox states(mHe[2]C^((8-n)·n+) and mHo[2]C^((8-n)·n+), where n refers to the numberof positive charges) from the corresponding cyclophanes were calculatedin MeCN at the M06/6-311++G** level taking previous reportedHo[2]C^((8-n)·n+) as the control molecule. For each of nine redoxstates, the theoretical association energy (ΔE) of mHe[2]C^((8-n)·n+)and mHo[2]C^((8-n)·n+) are always higher than the correspondingHo[2]C^((8-n)·n+), indicating that the introduction of mCBPQT ring into[2]catenanes is more energetically unfavorable than it is when the ringis CBPQT because of the smaller cavity size of mCBPQT ring.Nevertheless, if we consider the binding energy difference (ΔΔE) between7+ and 8+ state (ΔΔE = ΔE₇₊ - ΔE₈₊), or 6+ and 8+ state (ΔΔE = ΔE₆₊ -ΔE₈₊), a value which indicates the thermodynamic tendency of forming the7+ (monoradical) or 6+ (bisradical) states from the reduction of state8+, the two new mCBPQT-ring containing catenanes become energeticallymore favorable (Table S2) than that for Ho[2]C^((8-n)·n+). Accordingly,the mono- and bisradical forms of mCBPQT-ring-based [2]catenanes shouldexhibit enhanced stabilities compared with CBPQT-ring-based [2]catenane,an observation which is also in agreement with the experimental results.

Calculations were performed using density functional theory (DFT) withthe M06 functional, as implemented^(S5) in Jaguar 7.6.110. Geometryoptimizations were performed^(S6) using the 6-31G* basis set. Electronicenergies (Table 1) were obtained^(S7) using the 6-311++G** basis set. ΔEis the difference in energy between the sum of the individualmacrocycles of a particular change state and the corresponding[2]catenane of charge (8-n)·n+. Association energy differences (ΔΔE)detween different charged dtates are shown in Table 2. Solventcorrections were based on single point self-consistent Poisson-Boltzmanncontinuum solvation calculations for MeCN (ε = 37.5 and R₀= 2.179 Å)using^(S8) the PBF module in Jaguar.

TABLE 1 Calculated Association Energies (ΔE) of [2]Catenanes ChargedState ΔE (Ho[2]C) / (kcal mol⁻¹) ΔE (mHe[2]C) / (kcal mol⁻¹) ΔE(mHo[2]C) / (kcal mol⁻¹) 0 -29.0 -25.5 -20.6 1+ -42.3 -40.5 -35.6 2+-50.5 -48.7 -44.8 3+ -59.7 -58.2 -55.0 4+ -64.1 -60.0 -55.7 5+ -49.0-41.6 -39.0 6+ -26.8 -14.6 -12.5 7+ -0.40 9.3 13.5 8+ 40.2 55.6 58.9

TABLE 2 Calculated Association Energy Differences (ΔΔE) BetweenDifferent Charged States ΔΔE (Ho[2]C) / (kcal mol⁻¹) ΔΔE (mHe[2]C) /(kcal mol⁻¹) ΔΔE (mHo[2]C) / (kcal mol⁻¹) 8+ to 7+^(a) -44.2 -46.3 -45.48+ to 6+^(b) -67 -70.2 -71.4 ^(a)ΔΔE = ΔE₇₊-ΔE₈₊; ^(b)ΔΔE = ΔE₆₊-ΔE₈₊

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1. A composition comprising a [2]catenane, the [2]catenane comprising afirst macrocyclic ring mechanically interlocked with a secondmacrocyclic ring, or a salt thereof, wherein each of the firstmacrocyclic ring and the second macrocyclic ring comprise an alternatingcyclic arrangement of a first unsubstituted or substituted4,4′-bipyridinium (BIPY) subunit, a first unsubstituted or substitutedphenylene subunit, a second unsubstituted or substituted BIPY subunit,and a second unsubstituted or substituted phenylene subunit forming amacrocycle, wherein the first macrocyclic ring comprises ameta-phenylene subunit.
 2. The composition of claim 1, wherein thecomposition comprises an air-stable bisradical/hexacation or anair-stable monoradical/heptacation.
 3. The composition of claim 2,wherein the composition comprises the air-stable bisradical/hexacation.4. The composition of claim 1, wherein the first ring or both the firstring and the second ring are mCBPQT.
 5. The composition of claim 4,wherein both the first ring and the second ring are mCBPQT.
 6. Thecomposition of claim 4, wherein the first ring is mCBPQT.
 7. Thecomposition of claim 6, wherein the second ring is CBPQT.
 8. Thecomposition of claim 1, wherein the composition has an E_(red1) greaterthan +0.50 V versus Ag/AgCl.
 9. The composition of claim 1, wherein thecomposition has an E_(red2) greater than +0.25 V versus Ag/AgCl.
 10. Thecomposition of claim 1, wherein the composition has a near infraredabsorption band longer than 1200 nm.
 11. (canceled)
 12. A crystallinecomposition comprising the composition of claim 1 having a molecularpacking arranging defined by a triclinic, space group P1‾(no. 2) or aorthorhombic, space group Pna2₁ (no. 33).
 13. The crystallinecomposition of claim 12, wherein the composition has a molecular packingarranging defined by the triclinic, space group P1‾(no. 2) and latticeparameters of a = 14.1 ± 0.1 Å, b = 15.3 ± 0.1 Å, c = 23.3 ± 0.1 Å, α =84.6 ± 0.1°, β = 82.2 ± 0.1°, and γ = 66.5 ± 0.1°.
 14. The crystallinecomposition of claim 12, wherein the composition has a molecular packingarranging defined by the orthorhombic, space group Pna2₁ (no. 33) andlattice parameters of a = 27.2 ± 0.1 Å, b = 20.4 ± 0.1 Å, and c = 16.7 ±0.1 Å .
 15. The crystalline composition of claim 12, wherein thecomposition comprises six counter anions for every [2]catenane. 16.(canceled)
 17. (canceled)
 18. A method for preparing a [2]catenane, themethod comprising: (a) contacting a cationic ring with a cationic guestmolecule in the presence of a reducing agent, thereby reducing thecationic ring and the cationic guest molecule and forming a radicalcationic inclusion complex; and (b) reacting the guest molecule of theradical cationic inclusion complex with a ring-closing reagent toprepare the [2]catenane or reaching the termini of the guest molecule ofthe radical cationic inclusion complex with each other to prepare the[2]catenane.
 19. (canceled)
 20. The method of claim 18, wherein thecationic ring is mCBPQT⁴⁺.
 21. The method of claim 18, wherein thecationic guest molecule is

.
 22. The method of claim 18, wherein the cationic guest molecule is

.
 23. The method of claim 22, wherein the ring-closing reagent is4,4′-bipyridine.
 24. The method of claim 18 further comprising reducingthe [2]catenane with a second reducing agent to prepare a reduced[2]catenane.